Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles and hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel. As a result, kinds of applications using the secondary battery are being increased owing to advantages of the secondary battery, and hereafter the secondary battery is expected to be applied to more applications and products than now.
Secondary batteries have different structures depending upon outputs and capacities required by applications and products, to which the secondary batteries are applied. For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDAs), digital cameras, and laptop computers, use one or several small-sized, light cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, medium- or large-sized devices, such as electric bicycles, electric motorcycles, electric vehicles, and hybrid electric vehicles, use a battery module (or battery pack) having a plurality of cells electrically connected with each other because high output and large capacity is necessary for the medium- or large-sized devices. The size and weight of the battery module is directly related to the receiving space and output of the corresponding medium- or large-sized device. For this reason, manufacturers are trying to manufacture small-sized, light battery modules. Furthermore, devices, which are subject to a large number of external impacts and vibrations, such as electric bicycles and electric vehicles, require stable electrical connection and physical connection between components constituting the battery module. In addition, a plurality of cells are used to accomplish high output and large capacity, and therefore, the safety of the battery module is regarded as important.
Generally, a medium- or large-sized secondary battery module is manufactured by mounting a plurality of unit cells in a case (housing) having a predetermined size and electrically connecting the unit cells. A rectangular secondary cell or a pouch-shaped secondary cell, which can be stacked with high integration, is used as the unit cell. Preferably, the pouch-shaped cell is normally used as the unit cell, since the pouch-shaped cell is light and inexpensive.
However, the pouch-shaped cell, which is normally used as the unit cell of the battery module, has several problems in spite of the above-mentioned advantages.
First, the pouch-shaped cell has plate-shaped electrode terminals, which protrude from the upper end of the pouch-shaped cell. As a result, it is difficult to electrically connect the plate-shaped electrode terminals of the pouch-shaped cell, which is necessary to construct the battery module. Generally, the electrode terminals are connected with each other using wires, plates, or bus bars by welding. However, this electrical connection is difficult for the plate-shaped electrode terminals. For this reason, the plate-shaped electrode terminals are partially bent, and the plates or the bus bars are connected to the bent parts of the plate-shaped electrode terminals by welding, which requires skilled techniques. Also, this connecting process is very complicated. In addition, the connected parts may separate from each other due to external impacts, which results in increase in the number of defective products.
Secondly, the pouch-shaped cell has low mechanical strength. For this reason, additional members for maintaining stable connection and assembly are needed when a plurality of unit cells are stacked to manufacture a battery module. Consequently, when the battery module is manufactured using the pouch-shaped cells, unit cells are mounted in a cartridge, which is capable of receiving one or two unit cells, and a plurality of cartridges are stacked one on another so as to manufacture a battery module. As a result, a manufacturing process of the battery module is further complicated, and the size of the battery module is increased.